<?php
$aa = array(1,5,3,4,12,6,7,8,9,10,13);
print_r(getCombinationToString($aa,3));
function getCombinationToString($arr,$m)
        {
            $result = array();
            if ($m ==1)
            {
               return $arr;
            }

            if ($m == count($arr))
            {
                $result[] = implode(',' , $arr);
                return $result;
            }

            $temp_firstelement = $arr[0];
            unset($arr[0]);
            $arr = array_values($arr);
            $temp_list1 = getCombinationToString($arr, ($m-1));

            foreach ($temp_list1 as $s)
            {
                $s = $temp_firstelement.','.$s;
                $result[] = $s;
            }
            unset($temp_list1);

            $temp_list2 = getCombinationToString($arr, $m);
            foreach ($temp_list2 as $s)
            {
                $result[] = $s;
            }    
            unset($temp_list2);
            
            return $result;
        } 
?>